For enum types (described in Section 8.7), there are several
functions that allow cleaner programming without hard-coding
particular values of an enum type. These are listed in Table 9-28. The
examples assume an enum type created as:

Returns the range between the two given
enum values, as an ordered array. The values must be from
the same enum type. If the first parameter is null, the
result will start with the first value of the enum type.
If the second parameter is null, the result will end with
the last value of the enum type.

enum_range('orange'::rainbow,
'green'::rainbow)

{orange,yellow,green}

enum_range(NULL,
'green'::rainbow)

{red,orange,yellow,green}

enum_range('orange'::rainbow,
NULL)

{orange,yellow,green,blue,purple}

Notice that except for the two-argument form of enum_range, these functions disregard the
specific value passed to them; they care only about its declared
data type. Either null or a specific value of the type can be
passed, with the same result. It is more common to apply these
functions to a table column or function argument than to a
hardwired type name as suggested by the examples.